A process for pressing optically finished glass lenses is described in U.S. Pat. No. 4,481,023 (Marechal et al.). Optically finished glass lenses pressed in accordance with the process described therein have been marketed commercially by Corning Incorporated, Corning, N.Y., utilizing alkali metal fluoroaluminophosphate compositions disclosed in U.S. Pat. No. 4,362,819 (Olszewski et al.). The latter glass compositions are suitable for that purpose because they exhibit low transition temperatures (Tg) while demonstrating good chemical durability and resistance to weathering. Nevertheless, the presence of fluorine in the compositions gives rise to three serious practical problems. To illustrate:
As was explained in U.S. Ser. No. 599,743, supra, fluorine can attack the surfaces of the molds. Also, volatilization of fluorine during melting of the glass batch creates environmental hazards. Moreover, volatilization of fluorine from the surface of cooling glass bodies of the type described in U.S. Pat. No. 4,362,819 necessitates the removal of a fluorine-depleted surface layer therefrom through mechanical means prior to the pressing operation, which not only adds a time-consuming and expensive step to the process, but also exerts a negative impact on glass utilization.
Accordingly, the primary objective of the present invention was to devise glass compositions exhibiting properties similar to those demonstrated by the glasses disclosed in U.S. Pat. No. 4,362,819, but which would be free of fluorine.
More particularly, the primary goal of the present invention was to develop chemically durable, weather resistant, essentially fluorine-free glasses with annealing points within the temperature range of about 300.degree.-340.degree. C., thereby enabling them to be molded into lenses at temperatures on the order of 360.degree.-400.degree. C., and with refractive indices of about 1.605 and linear coefficients of thermal expansion (25.degree.-300.degree. C.) between 145-170.times.10.sup.-7 /.degree.C.